1. Field of the Invention
The present invention relates to a microdisplay pixel cell and method of making it, and more particularly, to a pixel cell with a metal-insulator-metal (MIM) capacitor.
2. Description of the Prior Art
In modern planar display technology, plasma display panel (PDP) and liquid crystal display (LCD) are two main streams. They both constitute numerous display grids called as pixel cells. The former one is applied in a large-sized market and still has a gap from widespread application since the technique for mass production is not broken through yet and cost is high. A thin-film transistor LCD (TFT LCD), which is prevailing in recent years, is a representative of the latter one and is mainly applied in the market smaller than 17 inches. During the fabrication of the TFT LCD products, however, defects such as dots or lines may occur to the LCD. Thus, compensate techniques are required to improve the production yields.
A microdisplay utilizes a silicon chip as a substrate and utilizes a standard CMOS process to form pixel cell matrices, integrated drivers and other electronic devices on the silicon chip. An advantage of the microdisplay is to utilize the CMOS process, since the CMOS process is well developed at the present semiconductor industry. As a result, high stability and reliability can be achieved when compared to the LCD. In addition, using this process, each pixel pitch can be shrank to less than 10 μm, therefore a high resolutions is obtained. When compared to the PDP, the microdisplay not only has an absolute superiority in cost but also has intrinsic advantages of the microdisplay. In addition, being assisted with adequate projection techniques, the microdisplay can further be applied in markets for large-sized displays. Therefore, a liquid crystal on silicon (LCOS) display, a kind of the microdisplays, attracts many major manufacturers to devote themselves in developing and is the display with highest potentiality.
Please refer to FIG. 1 of a layout of the prior art LCOS display pixel cell 10. The prior art LCOS display pixel cell 10 constitutes a transistor block 18, two pixel cap top plates 20 and one pixel cap bottom plate 22 disposed at either side of the transistor block 18. The pixel cap top plates 20 and the pixel cap bottom plate 22 form a pixel capacitor.
The transistor block 18 comprises four transistors 16. In other words, the transistor block 18 comprises two polysilicon gates 12 electrically connecting to a word line (not shown). Each polysilicon gate 12 crosses two active areas 14. One drain contact plug a is formed in each active area 14 for electrically connecting to a video data line (not shown), and two source contact plugs b are formed in each active area 14 for electrically connecting to a pixel cap top plate 20 respectively. One row select contact plug c is formed on top of each polysilicon gate 12 for electrically connecting to a row select line, which is the above-mentioned word line. Moreover, one contact plug d and one contact plug e are formed, respectively, on top of the pixel cap top plate 20 and the pixel cap bottom plate 22 for electrically connecting to the source contact plug b and ground.
Please refer to FIG. 2 to FIG. 7 of schematic diagrams of a method for forming the prior art liquid crystal on silicon (LCOS) display pixel cell 72. FIG. 2 to FIG. 7 are cross-sectional diagrams along line A–A″ shown in FIG. 1. As shown in FIG. 2, the prior art LCOS display pixel cell 72 is made on a semiconductor wafer 30. The semiconductor wafer 30 comprises a P-type silicon substrate 32. A plurality of isolators 34 are disposed on the surface of the P-type silicon substrate 32 for defining an active area for each device. The isolator 34 is usually a field oxide layer formed by a local oxidation (LOCOS) or a shallow trench isolation (STI).
AS shown in FIG. 3, a cleaning process is performed followed by homogeneously depositing a first polysilicon layer 36 on the P-type silicon substrate 32 utilizing a low pressure chemical vapor deposition (LPCVD) process. In the LPCVD process, silane (SiH4) is utilized as a reactive gas, a temperature is controlled in a range from 575° C. to 650° C., and a pressure is in a range from 0.3 to 0.6 torr.
Then as shown in FIG. 4, a first photoresist layer 38 is coated on the surface of the first polysilicon layer 36 followed by performing a first photolithography process to define two pixel cap bottom plate patterns 41 in the first photoresist layer 38. Thereafter a dry etch process is performed to vertically remove the first polysilicon layer 36 along the defined pixel cap bottom plate patterns 41 until reaching the surface of the isolator 34, so two pixel cap bottom plates 42 are formed. Following this, the first photoresist layer 38 is removed.
As shown in FIG. 5, a thermal oxidation is utilizeed to simultaneously form a gate oxide layer 44 composed of silicon dioxide (SiO2) on the surface of the active area 14 and simultaneously form a capacitor dielectric layer 45 on the surface of two pixel cap bottom plates 42. Then, a LPCVD process is performed to form a second polysilicon layer 46 on the surface of the P-type substrate 32 to cover the two pixel cap bottom plates 42.
Thereafter as shown in FIG. 6, a second photoresist layer 48 is formed on the surface of the second polysilicon layer 46. A second photolithography process is performed to define a gate pattern 49 and two pixel cap top plate patterns 51 in the second photoresist layer 48. Then, an anisotropic dry etch process is performed to remove the second polysilicon layer 46 not covered by the second photoresist layer 48 until reaching the surface of the gate oxide layer 44 and the capacitor dielectric layer 45, thus simultaneously form a transistor gate 52 and a pixel cap top plate 54. Finally, the second photoresist layer 48 is removed. The top plate 54, the bottom plate 42 and the capacitor dielectric layer 45 form a complete pixel capacitor 58.
As shown in FIG. 7, a third photoresist layer (not shown) is then formed and an ion implantation process is utilized to form a source/drain (S/D) 63 and 64 in the P-type substrate 32 at either side of the transistor gate 52. Thereafter, a dielectric layer 66 is formed on the P-type substrate 32. After that a photo-etching-process (PEP) is performed to form a plurality of contact holes 68 reaching the surface of the gate 52 in the dielectric layer 66, functioning as a row select contact plug c as shown in FIG. 1 to electrically connect to a subsequent formed row select line. After forming the contact holes 68, a chemical mechanical polishing process (CMP) is performed to the dielectric layer 66 to improve the planarization of the dielectric layer 66 and decrease the difficulty of a subsequent photo-etching-process for forming other contact holes.
Finally, back end processes, such as contact plug processes and metal interconnects processes are performed, respectively, to form the row select line, the inter-metal dielectric (IMD), the drain contact plug a, the source contact plug b, the contact plug d, the contact plug e and the video data line to complete the LCOS display pixel cell 72.
Since the pixel capacitor is located at either side of the transistor and occupies the same plane as the transistor according to the prior art design, this design cause a considerable limitation to chip size shrinkage. In addition, noise of the device cannot be effectively reduced because the length of each metal line in the device cannot be shortened. Moreover, since both the pixel cap top plate and the pixel cap bottom plate are composed of polysilicon, mismatch and residue problems tend to occur due to the effect of nonuniformity between the etching process and process parameter, such as stop layer. This results in deviations from the designed electrical performance of the capacitor.
To improve the electrical performance of the capacitor, the pixel cap top plate and the pixel cap bottom plate may be made of metal. In this case, etching problems can be effectively improved and noise can also be reduced. However, new problems still occur. In a subsequent high temperature process, if there is oxygen in the environment, the metal surface will be oxidized to induce peeling at the interface of the metal and other materials. Moreover, since the transistor gate is formed in the etching process of the second polysilicon layer, the reserved site for gate suffered two etching processes before the gate is complete and thus the quality of the gate is more or less affected. Therefore it is very important to develop a new LCOS display pixel cell design and a process in conjunction with the new design to resolve the above-mentioned problems.